1. Field of the Invention
The present invention generally relates to a multi-color image forming apparatus having a capability of reproducing an image of an original document in a plurality of colors.
2. Description of the Prior Art
The multi-color image forming apparatus is a machine having a plurality of developing units disposed in the vicinity of an electrostatic latent image carrier and accommodating powdery developers of different colors one for each unit, the developing units being selectively brought into operation so that an image of an original document can be reproduced in any one of the colors.
With the multi-color image forming apparatus widespread today, it has become desired that, where the original document to be reproduced includes, other than a text, tables and/or photos, a copy or reproduction of the image of the original document can be made having portions corresponding in position to the tables and/or photos which are printed or reproduced in a color different from that of the remainder thereof for the purpose of emphasizing the tables and/or photos.
To cope with the desire, the assignee of the present invention has filed the Japanese Patent Application No. 62-15572 directed to a multi-color image forming apparatus capable of making a copy having predetermined portions reproduced in a plurality of selected colors in one cycle of electrophotographic image forming process.
Generally, with the image forming apparatus, a reproduced image can be obtained having a uniform image density selected by an operator of the apparatus before the image of the original document is scanned. However, in the case of the multi-color image forming apparatus of a type wherein a single cycle of scanning operation suffices for the multi-color image to be formed, the uniformity of the image density would often bring about some inconveniences.
By way of example, where the image of the original document has different portions, one portion bearing an photo and the other portion bearing a text, and if the density of the image of the original document as a whole expected to be reproduced is so selected that the eventually reproduced image of the photo portion can exhibit an acceptable tone, the image of the text portion will be reproduced pale. Conversely, if the selection of the density of the image expected to be reproduced is made in reference to the density of the text portion so that the text portion can exhibit an acceptable tone, the eventually reproduced image of the photo portion will not exhibit an acceptable tone.
While the adjustment of the density desired to be attained in the eventually reproduced image can be accomplished by adjusting the amount of light used to scan the original document and/or the amount of toner applied to the electrostatic latent image carrier, it is frequently experienced with the multi-color image forming apparatus that, for a given amount of light used to scan the original document, the eventually reproduced image as a whole will show an image density varying from portion to portion because the electrical characteristic, for example, the electrostatic charge property, varies from toner of one color to that of a different color. Again, it is often recognized rather desirable that, due to the difference in color quality, for example, a red-colored portion of the original document be reproduced somewhat pale as compared with the density of that portion in the original document. somewhat pale.
In view of the foregoing, the prior art is such that, when it comes to the color reproduction of the original document having text and photo portions with the use of the multi-color image forming apparatus, either the density of the eventually reproduced image of one of the text and photo portions has to be sacrificed, or a compromise has to be made between the densities of the respective images of the photo and text portions in such a way as to choose an average image density.